Lunar and Planetary Science XXX 1226.pdf

EXPERIMENTAL DETERMINATION OF SIDERITE (IRON CARBONATE) STABILITY UNDER MODERATE PRESSURE-TEMPERATURE CONDITIONS, AND APPLICATION TO MARTIAN CARBONATE PARAGENESES. Andrea M. Koziol. Department of Geology, The University of Dayton, 300 College Park, Dayton, OH 45469-2364, USA; e-mail: [email protected].

Introduction: Siderite (FeCO3) occurs in fugacity of the experiments is defined to be on metamorphosed iron formations, altered igne- the hematite-magnetite buffer at that pressure ous rocks, and as a solid solution component and temperature. This is an appropriate fO2 in Fe-Mg carbonates that are found in car- value for application of the results to Earth bonatites [1, 2] . Its stability is controlled in parageneses as well as Martian carbonates. part by O2 fugacity and CO2 fugacity. Experimental Method: Experiments on Siderite therefore is of interest as a monitor reaction (2) were made in a piston-cylinder phase of these parameters. apparatus, using procedures similar to those An unusual extraterrestrial occurrence of described in [6] . Samples consisted of ho- carbonate has also focused attention on siderite mogenized powders of synthetic siderite, and Fe-Mg carbonates. The carbonate glob- hematite, and a natural magnetite, sealed in 1 ules in ALH84001 that have intrigued the sci- mm diameter Pt tube segments with weighed entific community are magnesite-siderite solid amounts of silver oxalate (Ag2C2O4) as the solutions with variable Ca content [3 - 5] . CO2 source. These piston-cylinder experi- Compositional zoning generally varies from ments were performed with the samples buff- Ca-rich cores, to magnesite-rich rims. In ered with hematite + magnetite mixtures. The some samples there is an iron-rich (siderite- buffer lasted throughout the length of the ex- rich) zone outside the magnesite rims. periment and weight loss data indicates all gas Although some experimental data on mag- in the samples was CO2. Two methods were nesite has recently become available [6] the used to monitor reaction direction. Powder data on siderite stability are limited and contra- X - ray diffractometer scans of the same solid dictory [7, 8] and experimental data on ternary charges obtained before and after the experi- carbonates are also contradictory [9, 10] . ments were compared. In addition weight loss While it is probable that non-equilibrium proc- upon puncturing of the sample capsule was esses related to variable fluid composition, low recorded. A CO2 yield at least 15% larger temperatures or impact processes have oper- than expected from the Ag2C2O4 decomposi- ated on Martian rocks, it is important to have tion indicated reaction to magnetite + CO2; a an understanding of the equilibrium properties CO2 yield 15 % smaller than expected signi- of siderite and other carbonate components, fied siderite and hematite growth. especially under changing temperature, pres- Results: Experiments on reaction 2 are sure, oxygen fugacity, and CO2 fugacity con- shown in Figure 1. These include all experi- ditions. ments in which inner and outer capsules The reaction of siderite with oxygen is maintained their integrity. In most experi- ments X-ray analysis confirmed the weight- 3 FeCO3 + 1/2 O2 = Fe3O4 + 3 CO2 (1) loss data. Experiments where change in vola- tile content or X-ray reflection intensity was (Fe3O4: magnetite). An easier way of per- not significant are considered as "no reaction". forming experiments on siderite in such a sys- No new phases were detected in the experi- tem is to use an appropriate mineral phase to mental charges. deliver the oxygen component: Discussion: Chai and Navrotsky [11] show that calculated decarbonation curves for FeCO3 + Fe2O3 = Fe3O4 + CO2 (2) siderite are shifted by 70 to 100° C (at 7 to 10 kbar), depending on the value used for DH°f (Fe2O3: hematite). Through the presence of of siderite. This adds greatly to uncertainties hematite and magnetite in the experimental in thermobarometric calculations for metamor- charge, and the addition of hematite and mag- phosed iron formations, carbonatites, and netite surrounding the charge, the oxygen other parageneses. Lunar and Planetary Science XXX 1226.pdf

DETERMINATION OF SIDERITE STABILITY: A. M.Koziol

A value of the enthalpy of formation Acknowledgments: Experiments were per- (DH°f) of siderite of -760.6 kJ (kilojoules) was formed in the laboratory of Dr. R.C. Newton, derived from the present reversal data. The University of Chicago. This abstract was par- data set of Holland and Powell [12] version tially supported by NSF grant #EAR9805873. 2.3 was used for values of entropy, heat ca- pacity, volumes of all phases and for the en- Figure: Pressure-temperature diagram thalpies of hematite and magnetite. showing experimental determination of the re- Work is underway to examine the reaction action siderite + hematite = magnetite + CO2. products of such experiments to see if textural Open symbols: siderite + hematite stable. relations involving voids or minute magnetite Filled symbols: magnetite + CO2 stable. Also grains are similar to those observed in ALH shown is the calculated curve using the dataset 84001 [13 - 15] . Brearley [14] has hypothe- of Holland and Powell (12) . sized that thermal decomposition of Fe-rich carbonate produced the concentration of tiny magnetite particles seen by McKay et al [15] and no biological hypothesis is necessary. This study can provide a framework of what equilibrium thermodynamics predicts for 14 carbonates of this range of compositions co- sid + hem existing with orthopyroxene and small grains of magnetite and SiO2, as is seen in 12 ALH84001 [4, 15] . It is possible that the chemical zoning seen in these carbonates can be interpreted as changes in temperature, CO2 10 mt + CO or oxygen fugacity in the vicinity of the car- 2 bonate globules. References: [1]. Buckley, H.A., Woolley, 8

A.R., (1990) Mineralogical Magazine 54, 413- Pressure, kbar 418 . [2] Butler, P. J., (1969) Journal of Pe- trology 10, 56-101. [3] Harvey, R.P., 6 Holland & McSween, H.Y. (1996) Nature 382, 49-51. [4] Mittlefehldt, D.W., (1994) Meteoritics 29, Powell 214-221. [5] Valley, J. W., et al., (1997) 4 Science 275, 1633-1638 . [6] Koziol, A. M., Newton, R. C. (1995) American Mineralogist 400 500 600 700 800 80, 1252-1260. [7] French, B. M. (1971) Temperature, °C American Journal of Science 271, 37-78. [8] Weidner, J. R. (1972) American Journal of Science 272, 735-751. [9] Goldsmith, J. R., Graf, D. L., Witters, J., Northrop, D. A. (1962) Journal of Geology 70, 659-688. [10] Rosenberg, P. E., (1967)American Mineralo- gist 52, 787-796. [11] Chai, L., Navrotsky, A. (1994) American Mineralogist 79, 921- 929. [12] Holland, T. J. B. , Powell, R. (1990) Journal of Metamorphic Geology 8, 89-124. [13] Bradley, J. P., Harvey, R. P., McSween, H. Y. J. (1996) Geochimica et Cosmochimica Acta 60, 5149-5155. [14] Brearley, A. J. (1998) LPSC XXIX ((CD- ROM), Abstract # 1451. [15] McKay, D. S., et al., (1996)Science 273, 924-930.